Multiple position cylinder

ABSTRACT

A multiple position cylinder having a cylinder housing, a primary piston assembly, and a plurality of secondary piston assemblies of varying stroke interlocked and axially aligned in end-to-end relationship between the primary piston assembly and the end of the cylinder, the length of stroke of all succeeding piston assemblies being equal to or less than the length of travel of all preceding piston assemblies.

United States Patent Gaitten Oct. 14, 1975 MULTIPLE POSITION CYLINDER 3,633,465 1 1972 Puster 92/133 [75] Inventor: W. Maynard Gaitten, Sylacauga, FOREIGN PATENTS OR APPLICATIONS 1,223,258 6 1960 Germany 91/167 1,576,079 4/1965 Germany 91/167 [73] Asslgnee' a i g g ggz z 513,477 10 1939 United Kingdom 91 167 19 g 937,859 9/1963 United Kingdom 92/62 [22] Filed: July 11, 1974 Primary Examiner-William R. Cline [2]] Appl' 4876l7 Assistant Examiner-Abraham Hershkovitz Related US. Application Data Attorney, Agent, or FirmFinnegan, Henderson, [63] Continuation of Ser. NO. 265,464, June 23, 1972, Farabow & Garrett abandoned.

I 91 167 R 92 62 92 5 [57] ABS CT [52] C l g 5 2 A multiple position cylinder having a cylinder housing, [51 J Int Cl 2 F1 5B 11/18 a primary piston assembly, and a plurality of second- Fieid 13 41 13 7 ary piston assemblies of varying stroke interlocked 92/61 62 6 and axially aligned in end-to-end relationship between the primary piston assembly and the end of the cylin- References Cited der, the length of stroke of all succeeding piston assemblies being equal to or less than the length of UNITED STATES PATENTS travel of all preceding piston assemblies. 2,699,757 l/l955 Tomkvist et a1 91/167 3,264,947 8 1966 Bidlack 91 /167 11 Clalms, 3 Drawmg Flgures 38C 36C 7413 38B 36B 74A 80 38A 36A 74 7 20 A/ 1 ll i MULTIPLE POSITION CYLINDER This is a continuation of application Ser. No. 265,464, filed June 23, 1972, now abandoned.

This invention relates to a multiple position cylinder and more particularly to a novel, fluid-actuated cylinder having a plurality of interlocking pistons of varying stroke forpositioning a controlled device or element in a more accurate and trouble-free manner.

Many devices have been provided in the past for digital positioning of a tool or element, such as a machine tool with respect to a work piece. Such devices range from simple hand-operated or template-guided mechanical devices to the more accurate and automatic, fluid-actuated devices in which the tool is located in numerous positions by the controlled force of the actuating fluid.

Such pneumatic or hydraulic operated fluid devices include double acting pistons, wherein the force differential in the cylinder controls the position of the piston with respect to the cylinder and ultimately the tool connected to the piston. A particular disadvantage with a double acting piston as a multiple position device, however, is that the piston is locked in position only through opposing fluid pressure, and in heavy machine tool work the forces created between the tool and the work often will overcome the fluid pressure and throw the tool out of alignment.

To overcome this disadvantage, particularly when large opposing forces are encountered in the operation of the controlled tool, it has been proposed to utilize a plurality of fluid-actuated cylinders, such as hydraulic cylinders, each having a piston stroke of varying length and stacked in end-to-end relationship to provide a more rigid connection between the controlled tool and the positioning device.

Exemplary of a stacked cylinder, multiple positioning device is the apparatus shown in U.S. Pat. No. 3,633,465 to Louis M. Puster. In the Puster device the pistons of the actuated cylinders slide all preceding cylinders their lengths of stroke so that the controlled tool conventionally attached to the piston of the first cylinder will be moved the total sum of the length of strokes of the actuated cylinders. By varying the length of strokes of the cylinders, it can be seen that a large variety of positions for the tool can be achieved.

While the use of a plurality of stacked cylinders has provided a more rigid connection between the controlled tool and the positioning device, it has not proven entirely satisfactory in all applications of work. The use of sliding cylinders, for example, requires each to have its own separate fluid entry port and separate hydraulic fluid line that moves with the cylinder to sup ply the cylinder with fluid regardless of its position. In a heavy machine operation, such as the positioning of a saw with respect to a log in a saw mill, these moving hydraulic fluid lines not only are hazardous to operating personnel, but are very susceptible to rupture through interference with other machine parts as they move back and forth positioning the saw in the desired location.

Further, whether the cylinders are permitted to slide on an open base or enclosed within a housing, as shown in Puster, dust, dirt and other foreign particles interfere with the sliding cylinders and the operation of the device. In saw mill operations, for example, multiple stacked cylinders have been abandoned as a multiple positioning device because they are next to impossible to keep free of wood chips and dust and in proper working order.

Another disadvantage of multiple stacked cylinders is that the effective force of the fluid is limited to the cross sectional area of the individual pistons and not the area of the housing, requiring more space to accommodate the device without realizing an increase in the effective operating pressure.

It is, therefore, a primary object of this invention to provide a new and improved fluid-actuated multiple positioning device that provides a rigid connection between the controlled tool and the positioning device while eliminating the foregoing and other disadvantages heretofore encountered in fluid operated, multiple positioning apparatus.

To achieve this object and in accordance with its purpose, this invention as embodied and broadly described provides a multiple position cylinder comprising:

a. a cylinder housing having a front end and rear end;

b. a primary piston assembly having a piston head sealably mounted for sliding movement within the cylinder and a piston rod extending outwardly of the front end of the cylinder;

a plurality of secondary piston assemblies axially aligned between the primary piston assembly and the rear end of the cylinder, each piston assembly having a piston head sealably mounted for sliding movement within the cylinder and defining a fluid tight chamber with the piston head on either side, the most rearward piston head defining a chamber with the rear end of the cylinder;

d. means interlocking the piston assemblies in end-toend relationship and limiting the length of travel of said piston assemblies, the most rearward piston assembly being interlocked with the end of the cylinder and the total length of extended travel of all succeeding piston assemblies being equal to or less than the length of extended travel of all preceding piston assemblies;

. a plurality of fixed fluid inlet ports in the cylinder housing, each port communicating with one of said chambers over the total length of travel of all succeeding piston assemblies;

. means for selectively sypplying fluid to said chambers whereby each actuated piston assembly slides all preceding piston assemblies the length of travel of said actuated assembly so that the length of travel of the primary piston assembly is equal to the cumulative total of the actuated piston assemblies; and

g. means for returning said piston assemblies to their initial retracted position.

The accompanying drawings which are incorporated in and constitute a part of the specification illustrate a preferred embodiment of the invention and together with the description serve to explain the principles of the invention.

Of the drawings:

H6. 1 is a cross sectional view of a multiple position cylinder constructed in accordance with the present invention with all the interlocked piston assemblies in retracted position;

FIG. 2 is a schematic cross sectional view similar to H6. 1; illustrating the primary piston assembly in actuated condition; and

FIG. 3 is a view similar to FIG. 2, but showing an additional piston assembly in actuated condition.

Reference will now be made in detail to the present preferred embodiment of the invention, the example of which is illustrated in the accompanying drawings.

With reference to FIG. 1, the multiple position cylinder of this invention includes a cylinder housing having a front end 12 and a rear end 14. A primary piston assembly, generally 16, including a piston rod 18 and a piston head 20 is assembled into cylindrical housing 10. Primary piston head 20 is sealed for sliding movement within cylinder 10 by a pair of double ring seals 22 and piston rod 18 extends outwardly through a cap 24 attached to the front end 12 of cylinder 10. The tool or element to be positioned by the cylinder is conveniently attached to or controlled by movement of primary piston rod 18 with respect to housing 10.

Cap 24 is secured to cylinder housing 10 in any suitable manner (not shown) and includes an O-ring seal 26 to provide a sealed fitting between the cap and the housing, an internal bearing 28 for piston rod 18, and an annular piston rod seal 30 to provide a pressure type housing while permitting sliding movement of piston rod 18 through cap 24. The rear end 14 of housing 10 similarly includes a rear cap 32 including an O-ring seal 34 to provide a sealed fitting between the cap and the rear end of the housing.

In accordance with the invention, and to provide a multiple position fluid actuated cylinder, a plurality of secondary piston assemblies are axially aligned and connected in end-to-end relationship between primary piston head 20 and end cap 32 of cylinder 10. With reference to FIG. 1, and by way of example only, there is shown a multiple position cylinder having six secondary piston assemblies, generally 36A, 36B, 36C, 36D, 36E, and 36F, and, in addition to primary piston assembly 16, provide six separate pistons of varying lengths of travel for operation or movement of primary piston rod 18. As more fully described below in connection with the detailed description of the cylinder, the total length of travel of all succeeding piston assemblies moving from front to back of the cylinder should be equal to or less than the length of travel of all preceding piston assemblies.

While the invention will be described with respect to the particular cylinder shown in the drawings, it will, of course, be appreciated by those skilled in the art that a cylinder of any number of secondary piston assemblies and varying lengths of travel can be provided without departing from the scope of the present invention.

While each piston assembly is provided with a different length of travel, each is constructed in the same manner and, therefore, similar reference numerals will be used in describing the construction ofthe secondary piston assemblies unless separate identification by letter is needed to differentiate between them. As shown in FIG. 1, each secondary piston assembly 36 includes a piston head 38, sealably mounted by a double ring lip seal 40 for sliding movement within cylinder housing 10. The secondary piston heads 38 are spaced along the length of cylinder 10 between primary piston head 20 and the rear end 14 of the cylinder defining a plurality of fluid tight chambers, generally 42, between the piston heads and between the rear most piston head 38F and the rear of the cylinder.

In accordance with the invention, means are provided for interlocking the piston assemblies in end-toend relationship within cylinder 10 for limiting the length of travel of each piston assembly, the most rearward piston assembly 36F being interlocked with the rear end 14 of the cylinder. As embodied, and as best shown in FIG. 1, the interlocking means comprises a cap, generally 44, having a forward guide plate portion 46 with a central aperture 47 and a cylindrical wall portion 48. Caps 44 are attached to the forward face 50 of each secondary piston head 38 by threaded boits 52 or other suitable means. A similar cap 45 is also attached to the rear cap 32 of cylinder housing 10.

Preferably, each cap 44 in the area of guide plate portion 46 is provided with a centering ring guide 68 housed in a depression for maintaining the secondary piston assemblies in axial alignment but is provided with a means, such as a cut-out 72, to permit fluid communication between both sides of the ring and prevent sealing at this point in the hydraulic cylinder.

The interlocking means also comprises a stem 54 extending axially and rearwardly of each secondary piston head 38, as well as a stem 56 of primary piston head 20, stems 54 and 56 extending through apertures 47 in the caps 44 ofthe next succeeding piston assembly, and the last stem 54F through cap 45. Thus, stem 56 of primary piston assembly 16 extends into the interior 58 of cap 44A, stem 54A into the interior 58 of cap 448, etc., the rear end 60 of each stem seating in a compatible depression 62 in the forward face 50 of each secondary piston head 38. A similar depression 64 is also provided in end cap 32 for receipt of the end 60 of piston stern 54F.

In accordance with the invention, stop means are provided for limiting movement of stems 54 and 56 within caps 44 and 45 and, hence, the length of its attached piston head 38 and 20 with respect to the next succeeding piston assembly. As embodied, the stop means comprises an annular ring 66 attached to stem 54 of each secondary piston assembly 36 as well as to stem 56 of primary piston assembly 16 and spaced slightly from the face 50 of piston heads 38 and the face 51 of rear cap 32 when the preceding piston assembly is in retracted position. To limit the length of travel, rings 66 are of larger diameter than apertures 47 in guide plates 46.

For each secondary piston assembly 36 as well as for primary piston assembly 16 there is provided a fixed fluid inlet port 74 communicating a source of hydraulic or pneumatic fluid with each one of the chambers 42 between the piston heads over the length of travel of all the succeeding piston assemblies. To permit communication between each chamber and its fixed fluid inlet port, the cap of each secondary piston assembly includes an annular axially extending groove 76 around the outer surface of the cylindrical wall portion 48 of each cap 44 and having a length 78 greater or equal to the length of travel of that and all succeeding piston assemblies. A port 80 in cap 44 communicates groove 76 with the interior 58 of the cap and aperture 47 in guide plate 46 communicates the interior of the cap with the rear face of the preceding piston head. Further, slot 72 in centering ring 68 also communicates fluid inlet 74 with the space in chamber 42 between the face of cap 44 and the preceding piston head 38.

Preferably, and as best shown in FIG. 1, the multiple position cylinder of the present invention is constructed so that the length of travel of each piston assembly, between its retracted position when the stems 54 and 56 seat in the forward face of the succeeding piston head 38 or end cap 32 and in its extended position when stop ring 66 abuts the inside surface of guide plate 46, is decreased by 50% in each succeeding piston assembly, so that the combined length of travel of all succeeding piston assemblies will be less than the length of travel of the preceding piston assemblies. Thus, the distance between the forward face of stop ring 66 on primary piston stem 56 and the rear face of guide plate 46 of the first secondary piston assembly 36A could be set, for example, to be four inches when the primary piston assembly is retracted; the same distance in the succeeding piston assembly 388 at 2 inches when the piston assembly 38A is retracted; in 38C, at 1 inch; in 38D, at one-half inch; in 38E, at one-quarter inch; in 38F, at one-eighth inch; and in rear cap 45, at one-sixteenth inch. Thus, in this example, the length of travel of all succeeding piston assemblies, for example, 38C, 38D, 38E, 38F and rear cap 45 add up to less than 2 inches which is less than the length of travel of that or any preceeding piston assembly.

In accordance with the invention, means are further provided for returning the piston assemblies to their initial retracted positions. As embodied, this means comprises a fluid inlet port 90 in cylinder housing communicating with the chamber 92 in front of primary piston head 20. Preferably, the pressure of the fluid supplied to chamber 92 is the same as that applied to the piston assemblies, but since the area of the forward face 91 of piston head is less than the area of the rearward faces of all the piston heads, the effective forward force of the pressure applied to the rear faces of pistons will overcome the force of pressure in chamber 92 sufficient to extend the pistons the desired distance. Further the fluid is continuously supplied to chamber 92 to maintain all piston assemblies in retracted position except when the assemblies are actuated in a forward direction and so that when the piston assemblies are deactuated the force of the pressure in chamber 92 will automatically push the deactuated pistons back to their initial retracted position.

In operation, and with reference to FIGS. 2 and 3, suitable valves (not shown) with control means for the valves are provided to selectively supply fluid to the various fluid inlets 74 for the secondary and primary piston assemblies. With the multiple position cylinder in fully retracted position, as shown in FIG. 1, ifit is desired to move primary piston rod 18 4 inches forward of the cylinder, primary piston assembly 16 is actuated in a forward direction by supplying fluid to inlet 74 and the chamber 42 between secondary piston head 38A and primary piston head 20. As shown by the arrows, the pressurized hydraulic fluid passes through groove 76 and conduit 80 into the interior 58 of cap 44A of first secondary piston assembly 36A and through aperture 47 between primary stem 56 and guide plate 46 and against the rear face of primary piston head 20, overcoming the back pressure on primary piston head 20 and moving primary piston rod 18 forward of the cylinder the same distance. can be seen from the drawings, the hydraulic fluidacts against not only the rear face of the piston head, but also the rear end 60 of primary stem 56 after it unseats from the depression 62 in piston assembly 38A, thus providing a piston area equal to the full cross sectional dimension of the inside of the primary cylinder housing 10. All other secondary piston assemblies are, of course, unactuated and remain in their initial positions, as shown in FIG. 2, due to the force of the fluid being continuously supplied to chamber 92.

If it is then desired to move piston rod 18 back to its initial position, the forward force on primary piston assembly 16 is released so that the pressure in chamber 92 pushes the primary piston assembly back to its initial position, the hydraulic fluid passing back out through inlet 74 to sump.

Instead, for example, if it was desired to move primary piston rod 18 and the attached tool from its 4- inch spacing to a 5-inch spacing, then, leaving primary piston assembly 16 actuated in its forward condition, the l-inch piston assembly 36B is actuated in a forward direction by supplying fluid to inlet 74B and chamber 428 between secondary piston head 38C and 38B, extending piston assembly 368 forward 1 inch in the same manner as that described in connection with primary piston assembly 16. The actuation of secondary piston assembly 368 slides preceding piston assemblies 36A and 16 also forward 1 inch, so that primary piston rod 18 will now be extended 1 inch forward of its former position, or 5 inches from its initial retracted position.

Thus, it can be seen that with selective actuation and deactuation of various combinations of the piston assemblies, the location of piston rod 18 and the attached tool can be made equal to the cumulative total of the actuated pistons.

It will be apparent from the foregoing description that this invention provides a new and unique multiple position cylinder comprising a plurality of interlocking pistons capable of being selectively actuated to provide a length of movement of the cylinder equal to the cumulatjve total of the individual actuated pistons. By providing a plurality of interlocking pistons, as opposed to a plurality of interlocking cylinders, heretofore known in the art, the need for movable hydraulic flow lines has been eliminated and the pressure supplied to the cylinder is effective over the cross sectional dimension of the cylinder. By eliminating the need for moving hydraulic flow lines, not only has a dangerous and hazardous aspect of previous multiple position devices been eliminated, but the ability of foreign particles, such as wood chips in a saw mill, to interfere with the operation has also been substantially eliminated. It should further be appreciated that this has been accomplished without eliminating the rigid connection between the individual moving parts of the cylinder, whether they are in actuated or deactuated condition, to provide a stronger and more accurate position cylinder.

The invention in its broader aspects is not limited to the specific details shown and described and departures may be made from such details without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

l. A multiple position cylinder comprising:

a. a cylinder housing having an internal bore and a front end and a rear end;

b. a primary piston assembly having a piston head, means for sealably mounting the piston head for sliding movement within the cylinder bore, and a piston rod extending outwardly of the front end of the cylinder;

a plurality of secondary piston assemblies axially aligned between the primary piston assembly and the rear end of the cylinder, each piston assembly having a piston head, means for sealably mounting each secondary piston head for sliding movement within the cylinder bore, each piston head defining with the cylinder bore a fluid-tight chamber with the piston head on either side, the most rearward piston head defining a chamber with the rear end of the cylinder and each piston head having a forward and a rearward face, a stem fixedly connected to and extending axially and rearwardly from the rearward face of the primary piston head and each secondary piston head, said stem having an end terminating in the chamber between said piston head and the next succeeding piston head, whereby fluid admitted to any one of said chambers acts on a surface area substantially equal to the cross-sectional area of the internal bore of the cylinder housing, said surface area comprising the rearward face of the piston head and the end of the stem;

d. means interlocking the stem of the piston of the immediately preceding piston assembly with the piston of the next succeeding piston assembly to interlock the piston assemblies in end-to-end relationship and limit the length of travel of each piston assembly, further means interlocking the most rearward piston assembly stem with the end of the cylinder housing and the total length of extended travel of all succeeding piston assemblies being equal to, or less than, the length of extended travel of each preceding piston assemblies, said means for interlocking the piston assemblies comprising a guide plate spaced from and connected to the forward face of the piston head of each secondary piston assembly and. a guide plate spaced from and connected to the rear end of the cylinder, said plates having a central aperture with the stem of the preceding piston head extending rearwardly through the aperture in said guide plate, stop means on each stem located behind the guide plate to limit the length of forward travel of the stem with respect to the guide plate and hence the attached piston assembly with respect to the next succeeding piston assembly, and a guide ring around the outer surface of each of the guide plates connected to the secondary piston heads that engages the cylinder bore to maintain the secondary means for selectively supplying fluid to said champiston assemblies in axial alignment within the cylinder;

. a plurality of fixed fluid inlet ports in the cylinder,

bers whereby each actuated piston assembly slides all preceding piston assemblies the length of travel of said actuated assembly so that the length of travel of the primary piston rod is equal to the cumulative total of the actuated piston assemblies; and

g. means for returning and maintaining said piston assemblies in their initial retracted position except when fluid is supplied to said chambers.

2. The cylinder of claim 1, wherein the stop means comprises a stop ring connected to the stem of each piston and abutting the rear face of the guide plate when the piston assembly is extended with respect to the guide plate of the next succeeding piston assembly.

3. The cylinder of claim 2, wherein the rear end of the stem abuts the forward face of the piston head of the next succeeding piston assembly when the preceding piston assembly is retracted.

4. The cylinder of claim 3, including a depression in the forward face of each piston head for seating the stem of the retracted piston assembly.

5. The cylinder of claim 2, wherein the stop ring is spaced from the forward face of the piston head of the next succeeding piston assembly when the piston assembly is retracted.

6. The cylinder of claim 1, including a cylindrical sleeve connecting the guide plate to the forward face of the piston head.

7. The cylinder of claim 6, wherein the sleeve and guide plate form a cap attached to the forward face of each secondary piston head, said cap having a port communicating the cylinder inlet port with the interior of the sleeve and the central aperture communicating the interior of the sleeve with the rear face of the piston head of the next preceding piston assembly thereby permitting fluid communication throughout said chamber between the adjoining piston heads.

8. The cylinder of claim 7, wherein the central aperture in the guide plate is larger than the stem to permit said fluid communication throughout the chamber.

9. The cylinder of claim 7, including an annular groove around the outer surface of said cap extending in an axial direction and communicating the port in the cap with the cylinder inlet port for that chamber over the total length of travel of all the succeeding piston assemblies.

10. The cylinder of claim 1, including means in said guide ring for maintaining a fluid communication throughout the chamber between adjoining piston heads.

11. The cylinder of claim 1, wherein the means for returning and maintaining the pistons in their initial retracted position comprises means for continuously supplying fluid to the forward face of the primary piston head. 

1. A multiple position cylinder comprising: a. a cylinder housing having an internal bore and a front end and a rear end; b. a primary piston assembly having a piston head, means for sealably mounting the piston head for sliding movement within the cylinder bore, and a piston rod extending outwardly of the front end of the cylinder; c. a plurality of secondary piston assemblies axially aligned between the primary piston assembly and the rear end of the cylinder, each piston assembly having a piston head, means for sealably mounting each secondary piston head for sliding movement within the cylinder bore, each piston head defining with the cylinder bore a fluid-tight chamber with the piston head on either side, the most rearward piston head defining a chamber with the rear end of the cylinder and each piston head having a forward and a rearward face, a stem fixedly connected to and extending axially and rearwardly from the rearward face of the primary piston head and each secondary piston head, said stem having an end terminating in the chamber between said piston head and the next succeeding piston head, whereby fluid admitted to any one of said chambers acts on a surface area substantially equal to the cross-sectional area of the internal bore of the cylinder housing, said surface area comprising the rearward face of the piston head and the end of the stem; d. means interlocking the stem of the piston of the immediately preceding piston assembly with the piston of the next succeeding piston assembly to interlock the piston assemblies in end-to-end relationship and limit the length of travel of each piston assembly, further means interlocking the most rearward piston assembly stem with the end of the cylinder housing and the total length of extended travel of all succeeding piston assemblies being equal to, or less than, the length of extended travel of each preceding piston assemblies, said means for interlocking the piston assemblies comprising a guide plate spaced from and connected to the forward face of the piston head of each secondary piston assembly and a guide plate spaced from and connected to the rear end of the cylinder, said plates having a central aperture with the stem of the preceding piston head extending rearwardly through the aperture in said guide plate, stop means on each stem located behind the guide plate to limit the length of forward travel of the stem with respect to the guide plate and hence the attached piston assembly with respect to the next succeeding piston assembly, and a guide ring around the outer surface of each of the guide plates connected to the secondary piston heads that engages the cYlinder bore to maintain the secondary piston assemblies in axial alignment within the cylinder; e. a plurality of fixed fluid inlet ports in the cylinder, each port communicating with one of said chambers over the total length of travel of all the succeeding piston assemblies; f. means for selectively supplying fluid to said chambers whereby each actuated piston assembly slides all preceding piston assemblies the length of travel of said actuated assembly so that the length of travel of the primary piston rod is equal to the cumulative total of the actuated piston assemblies; and g. means for returning and maintaining said piston assemblies in their initial retracted position except when fluid is supplied to said chambers.
 2. The cylinder of claim 1, wherein the stop means comprises a stop ring connected to the stem of each piston and abutting the rear face of the guide plate when the piston assembly is extended with respect to the guide plate of the next succeeding piston assembly.
 3. The cylinder of claim 2, wherein the rear end of the stem abuts the forward face of the piston head of the next succeeding piston assembly when the preceding piston assembly is retracted.
 4. The cylinder of claim 3, including a depression in the forward face of each piston head for seating the stem of the retracted piston assembly.
 5. The cylinder of claim 2, wherein the stop ring is spaced from the forward face of the piston head of the next succeeding piston assembly when the piston assembly is retracted.
 6. The cylinder of claim 1, including a cylindrical sleeve connecting the guide plate to the forward face of the piston head.
 7. The cylinder of claim 6, wherein the sleeve and guide plate form a cap attached to the forward face of each secondary piston head, said cap having a port communicating the cylinder inlet port with the interior of the sleeve and the central aperture communicating the interior of the sleeve with the rear face of the piston head of the next preceding piston assembly thereby permitting fluid communication throughout said chamber between the adjoining piston heads.
 8. The cylinder of claim 7, wherein the central aperture in the guide plate is larger than the stem to permit said fluid communication throughout the chamber.
 9. The cylinder of claim 7, including an annular groove around the outer surface of said cap extending in an axial direction and communicating the port in the cap with the cylinder inlet port for that chamber over the total length of travel of all the succeeding piston assemblies.
 10. The cylinder of claim 1, including means in said guide ring for maintaining a fluid communication throughout the chamber between adjoining piston heads.
 11. The cylinder of claim 1, wherein the means for returning and maintaining the pistons in their initial retracted position comprises means for continuously supplying fluid to the forward face of the primary piston head. 